With the rising cost of wood and the shortage of mature trees, there is a present need to find good quality substitutes for wood which continues long into the future. Over the past several years a growing market has emerged for the use of polymer-wood composites to replace traditional solid wood products in applications such as decking, windows, fencing, automobile interiors and pallets. These composite materials typically consist of mixtures of thermoplastic materials with wood particles in the form of sawdust. The composite materials may be used in many of the same applications as an all-wood product but offer the advantages of providing flame resistance, as well as enhanced resistance to rot, attack by insects, and deterioration due to the effects of moisture and sunlight. These products can have the same workability as wood, are splinter-free, and are capable of being colored in bulk as opposed to wood, which can only be surface stained or painted.
Recently there has been an increased interest in composites of wood and polyvinyl chloride (PVC), particularly for use in home siding applications. Such composites are highly desirable because they resemble traditional wood siding more than does uncomposited PVC siding. Moreover, compositing with wood raises the sag temperature of PVC and thus permits the use of dark colors in the composite siding. Dark colored, uncomposited PVC, with its lower sag temperatures, absorbs considerable heat in sunlight and exhibits a tendency to sag. See, e.g., U.S. Pat. Nos. 6,011,091; 6,103,791; and 6,066,680; and U.S. Patent Application 2003/0229160.
However, the known composites contain more than about 40 weight % (%) wood, particularly when they are extruded in a form suitable for siding applications, suffer from edge tear and slow, difficult extrusion.
It would be highly desirable, therefore, to have PVC/wood composites that comprise wood and still have physical properties that allow them to be made using typical PVC processes and be used in traditional PVC applications such as home siding.